thermocouples

[Darrell Norquay]

> I use the AD594 device to compensate and linearize thermocouples.  
> 
> Does anyone have experience multiplexing in front of one?
> 

John:

It's pretty difficult to multiplex the direct signals from thermocouples.
Using an electronic approach, any type of analog multiplexer usually has a
relatively high series resistance (compared to the t'couple) and introduces
a lot of error.  A mechanical approach (using relays) is better in some
respects, but you still get a thermocouple effect from the metal in the 
contacts, as well as resistance, and it's slow.  Mercury wetted reed relays
may work, I've never tried this.  In any case, you'd have to do the cold
junction comp before the MUX anyway.

A better approach would be to use some sort of a simple preamp circuit to
bring the output of each thermocouple up to a more manageable level, and
buffer the thermocouple output with a low impedence driver.  You could then
use a standard multiplexer chip and a final gain/offset stage to get a 
calibrated output.

The Analog Devices AD594 has no linearization of the TC input.  You can 
use some external components to improve the linearity, but a type K is
reasonably linear anyway, (better than other TC types) and for this 
application you don't really care if the temp is 1294.8 deg or 1298.4 deg, 
a few degrees one way or the other isn't going to make a difference.

Analog Devices has a relatively new chip, P/N AC1226, which does the 
cold junction comp, buffers the signal, and gives an output of 10 mV per
degree C.  It also has some rudimentary linearization built in, it's
pretty general but does improve the linearity significantly.  The device
has a built in temperature sensor, and is extremely low power to minimze
self heating, you would physically mount it right on your isothermal block
where TC leads meets copper wire.  I don't know the cost on these, but it 
should be reasonable.  If you use one per TC, and then MUX these together 
into an A2D or display device of some kind, you should be able to come up 
with a relatively low cost system with good accuracy.

If you don't feel up to building one, almost all manufacturers of PC A2D
converter boards (National Instruments, Strawberry Tree, Burr Brown, etc.) 
have an optional multi-channel signal conditioner / terminal board that 
have cold junction comp and preamps built in.  These usually run in the 
$150-$300 range, a bit dear but saves a lot of work, especially if you only
need one.

regards
dn
 

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 Darrell A. Norquay              Internet: dn at dlogtech.cuc.ab.ca     
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